Short version: The character the fandom calls "Tenka" doesn't actually have a known given name and "Tenka" is actually his title, similar to the Banken/Guard Dog and Kaibutsu/Monster situation.

Long version: Everyone other than Tenka himself, including the narrator, refers to him as "tenka-sama" (天下様), which is an archaic word for "Lord" or "Ruler", though the honorific suffix "-sama" is sometimes dropped.

When Unsai asks Tenka to state his name in volume 17, Tenka calls himself "The Emperor" ("mikado"/帝), but later on tells Unsai to refer to him as "tenka-sama" like all his other underlings do. Tenka is apparently the descendant of dogs who were known and respected as the emperors of dogs in Japan, and the whole lineage originated from ninja dogs who served directly under the (human) emperor of Japan, which is why he and his underlings consider him an emperor.
For what it's worth, the Finnish translation refers to him as "Valtias", which literally means "Ruler".
I'm not saying we should all stop calling him "Tenka" right this instant, but this might be worth keeping in mind for things like image gallery topics. Plus it's a neat little piece of Ginga trivia
